This application claims the benefit of U.S. Provisional Application No. 60/224,276, filed Aug. 10, 2000, which is incorporated by reference as if fully set forth herein for all purposes.
This invention relates to the use of a particular antistatic additive to produce dissipation of static electricity from a polymer host, in particular from ionomers.
Polymer films in the course of their handling can develop high charges of static electricity which can have harmful consequences ranging from sparking, possibly causing fires, to adhering to oppositely charged surfaces to interfere with use of the film in packaging operations. For example, film used to package food may be more advantageously handled in the packaging operation if the food is not attracted to the film. Film attraction for the food can prevent the desired wrinkle-free packaging of the food by the film and can lead to leakers in the final packaging seal.
A wide variety of antistatic compounds are available for incorporation into polymer films for reducing static electricity charges of the film, with varying efficacy depending on the polymer and the antistatic compound and the amount of the compound that can be tolerated in the film before suffering loss of other desired properties. A publication of ICI Specialty Chemicals entitled xe2x80x9cATMER(copyright) Antistatic Agent for Thermoplastic Polymer Applicationsxe2x80x9d (November 1987) discloses a large number of antistatic compounds.
An improvement in the antistatic performance of a film of ethylene/carboxylic acid copolymer, which may be partly neutralized to form ionomer, was described in U.S. Pat. No. 5,037,875 which provided a synergistic additive combination of sorbitan monolaureate and an alkyl phenol poly (ethylene oxide). Such combination additives were found to provide an improved static decay (about 1 week) compared to individual additives and other combinations (several weeks).
The synergistic additive claimed in U.S. Pat. No. 5,037,875 provides superior antistatic properties, but has the following drawbacks: it requires a certain amount of time until the antistatic properties have fully developed and it does not function in sodium ionomers.
All references set forth anywhere in this Specification are incorporated by reference as if fully set forth herein for all purposes.
The present invention overcomes the above-mentioned drawbacks. It involves the discovery that the use of a certain antistatic additive improves dissipation of static electricity in ethylene xcex1,xcex2-ethylenically unsaturated C3-8 carboxylic acid copolymers particularly those partially neutralized with sodium. More generally, the invention involves the discovery that a certain antistatic additive which surprisingly solves the above-indicated problems of prior art formulations, even more surprisingly provides an effective antistatic effect immediately after a sheet or film is produced, whereas the best antistatic additive combinations known from the prior art need at least about 1 week to develop any significant effect on surface resistivity.
According to the invention, a blend capable of being melt fabricated into an antistatic film comprises ethylene xcex1,xcex2-ethylenically unsaturated C3-8 carboxylic acid copolymers having 0-99% of the carboxylic acid groups neutralized by metal ions, particularly sodium ions, and an antistatic effective amount of polyoxyethylene sorbitan mono-C9-20 fatty acid, particularly ones having a viscosity at 25xc2x0 C. of less than 2000 mega Pascal.seconds (mega Pa.s) and less than 50 ethyleneoxide groups, most particularly Polyoxyethylene Sorbitan Monolaureate.
Film or sheet which is melt fabricated from this blend has superior antistatic properties as compared to film made of antistatic compounds described in U.S. Pat. No. 5,037,875, in particular in respect of producing an effective antistatic effect very soon after formation of a film or sheet.
The invention also covers the above-defined blend, melt fabricated into the form of a sheet or a film, supported or unsupported.
Such film when extruded at temperatures above 250xc2x0 C. and coated onto metal or onto a polymer film or paper displays outstanding antistatic properties. Other uses of the antistatic blend of the present invention include mono- or multi-layer films for packaging applications, sheet material incorporated in a floor tile, sheet material incorporated in auto interior components, and film incorporated in a package for dry powdery products (dry powdery package).
The invention also covers the above-defined blend further comprising at least one slip agent. By xe2x80x9cslip agentxe2x80x9d is meant herein an agent, particularly one capable of migrating to the surface of the sheet, that is effective in reducing the coefficient of friction between adjacent sheets or parts. Such slip agents are well know in the art and include fatty acid amides that are used in polyolefins present at a level up to 3% based on weight of the copolymer plus the antistatic agent.
The invention also covers the blend fabricated into a supported film that is surface treated such as by corona treatment after production to improve adhesion of the film to the substrate. Corona treatment is used to increase the surface energy of the film by generating therein carboxyl groups via oxidation of the polymers of the surface through electric discharge.
The copolymers used in the present invention are ethylene xcex1,xcex2-ethylenically unsaturated carboxylic acid copolymers or the ionomer obtained therefrom. Description of the preparation of these copolymers and the melt fabrication of film therefrom is provided in U.S. Pat. Nos. 4,248,990; 3,264,272 and 4,351,931. The copolymer can be random or non-random, but random is preferred. Preferred unsaturated acids contain 3 to 8 carbon atoms and include acrylic acid, methacrylic acid and itaconic acid. Preferably the copolymer contains 5 to 50% of the acid co-monomer based on the weight of the copolymer, and more preferably from 5 to 20 weight percent. The melt index of the copolymer is preferably less than 30 g/10 min. and more preferably less than 20 g/10 min measured in accord with ASTM D-1238, condition E, at 190xc2x0 C., using a 2160-gram weight.
In the case of ionomer obtained from these acid copolymers, the preferred metal ions for neutralizing the acid groups are Na+, K+, Li+, Mg2+, Ca2+ and Zn2+. The preferred neutralization is about 10 to 99% more preferably 15 to 50% of the acid groups present in the copolymer.
Thus, the preferred copolymer is an ionomer of ethylene containing 5-20 wt % methacrylic or acrylic acid neutralized 10 to 90% with sodium.
The antistatic agent used for the invention is a polyoxyethylene sorbitan mono-fatty acid, particularly one having a viscosity at 25xc2x0 C. of less than 2000 mega Pa.s, less than 50 ethyleneoxide groups, and a fatty acid chain length of from 9 to 20 carbon atoms. The most preferable antistatic agent is a polyoxyethylene sorbitan monolaureate that is commercially available from Croda France S.A. as xe2x80x9cCrillet(copyright) I Super.xe2x80x9d Polyoxyethylene sorbitan monolaureate is a surface active compound known as an internal or external antistatic agent for a great number of plastics, yet it is surprising that it has the utility found when used in the copolymer blend according to this invention. It could not be predicted that it would provide an almost instantaneous antistatic action, nor that it would solve the long felt problems relating to achieving desired antistatic properties with ethylene xcex1,xcex2 ethylenically unsaturated C3-8 carboxylic acid copolymers partially neutralized with sodium ions. These sodium-based ionomers are fundamentally different from zinc-based ionomers as, in the former, the sodium ions are more concentrated in the ionic clusters whereas, in the latter, the zinc ions are concentrated in the ionic clusters as well as in the matrix of the ionomer.
The antistatic agent is uniformly incorporated into the copolymer by conventional melt blending techniques, for example, the antistatic agent is blended with molten copolymer in a manner to form a homogeneous blend, using for example an extruder. The antistatic agent can be fed to the extruder along with the copolymer feed or can be metered into the copolymer as it advances as a melt along the extruder barrel. The resultant blend can be extruded and cut into granules for subsequent melt fabrication into antistatic film or can be directly fabricated into the film. Alternatively, the antistatic agent can be melt blended with the copolymer to form a concentrate of the antistatic agent for subsequent blending with the copolymer used in the present invention. In the concentrate approach, the polymer of the concentrate may be the same as the copolymer used in the subsequent blend or can be a different polymer which, however, is miscible with the copolymer upon melt blending.
The total amount of antistatic additive present in the ultimate blend used for making the antistatic film is about 0.5 to 3.0% based on the weight of the copolymer plus the weight of the additive, preferably about 0.75 to 2.5% and even more preferably about 1.0 to 2.0% by weight.
Very surprisingly, the antistatic action of the additive according to the invention is effective almost instantaneously, after less than 1 hour from when a film or sheet is produced, whereas control compounds need at least 1 week to develop any significant effect on reduction of surface resistivity, as witnessed by the best results reported in the aforementioned U.S. Pat. No. 5,037,875 or are not at all effective as with most of the antistatic agents in the case of Na ionomers if they are added at reasonable levels (such as 2 weight %)
This xe2x80x9calmost-instantaneousxe2x80x9d antistatic action is extremely important during production of films where operators are exposed with static built up on the extrusion equipment.
The described additive is useful in applications where dissipative properties are important, such as supported or unsupported sheets forming floor tiles and auto interior components, where use of the additive leads to the advantage that dirt and dust attraction, which is very annoying in these applications, is largely reduced.
The described additive can even be used as an additive for ionomer sealants to produce supported films for use in dry powdery packages where it serves to reduce the occurrence of powdery contamination of the seal area and leads to more reliable package seals.
The blend of the present invention can be melt fabricated into film by conventional methods. The film can be unsupported or it can be supported as in the case of forming a coating on a substrate. In the case of supported film, this can be made by such conventional methods as extrusion coating and co-extrusion. The resultant film, whether supported or unsupported will generally have a thickness of about 0.2 to 2 mils (0.005 to 0.05 mm). Examples of substrates for supporting the film of the present invention include paper, foil, and polyolefins.
The film is especially useful for packaging articles such as powders which themselves present a static charge that would otherwise attract an oppositely static charged film. One particularly annoying problem solved by the film of the present invention is that the filmxe2x80x94even shortly after its production and even if extruded at a temperature above 250xc2x0 C.xe2x80x94will not attract oppositely charged powder particles to the seal area of the film package.